1. Field of the Invention
The present invention relates to an etching method for a polysilicon film, and more particularly, to a dry etching method for a polysilicon film formed during the manufacturing process of a semiconductor device.
2. Description of the Prior Art
In recent years, semiconductor devices are being designed with increasingly three-dimensional features in their structures, along with the enhancement of the degree of integration. Because of that, the processes for film formation in parts with large step difference are increasing in number. So far as the polysilicon films are concerned, one that is used as an electrode of a stacked capacitor for a dynamic random access memory (DRAM) is formed on underlying material having the large step difference. A method of forming the lower electrode of a stacked capacitor that uses a polysilicon film will be described as follows.
First, after forming a field oxide film with thickness of about 300 nm on a silicon substrate by a selective oxidation method, a wiring that is connected to a gate electrode is formed on one end of the field oxide film using a first polysilicon film with thickness of about 500 nm. Next, after forming all over the surface a 150 nm thick silicon oxide film as an interlayer insulating film, a 600 nm thick second polysilicon film which is to become a lower electrode is formed on top of it. At this time, a step difference of about 800 nm is generated in the second polysilicon film due to the roughness of the underlying material.
Next, the second polysilicon film on the field oxide film is masked with a photoresist film so as to have an end of the photoresist film to be situated on the wiring of the first polysilicon film, and the exposed part of the second polysilicon film is etched by the reactive ion etching (RIE) method. When the etching (namely, RIE, and similarly in the following) of the flat part of the second polysilicon film is completed, there is left a residue of the second polysilicon film on the side faces of the wiring because the second polysilicon film is being formed thick. In order to remove the residue of the second polysilicon film completely, an etching time which is about twice as long is required. The RIE method is principally employed for etching the polysilicon film, and a chlorofluorocarbon gas such as CClF.sub.3, a bromochlorofluorocarbon gas such as CClBrF.sub.2, and a gas consisting mainly of Cl.sub.2 are frequently employed as the etching gas.
Further, the use of HBr gas as an etching gas for the polysilicon film based on its high selectivity with the oxide film formed on the semiconductor substrate and its high anisotropy has been reported by L. Y. Tsou in the Journal of the Electrochemical Society, Vol. 136, No. 10, 1989, p. 3003. In etching a polysilicon film using HBr gas, there is a case in which HBr gas alone is used or a case in which He is added to stabilize the plasma discharge. In either case, however, the selectivity of polysilicon to SiO.sub.2, the ratio of the etching rate of the polysilicon film to the etching rate of the oxide film, is higher than 100.
However, the conventional etching gases for a polysilicon film described in the above, a chlorofluorocarbon gas such as CCl.sub.2 F.sub.2 or a bromochlorofluorocarbon gas such as CClBrF.sub.2 has a small selectivity with respect to the oxide film on the order of 20. Therefore, if the second polysilicon film is overetched in order not to leave etching residue, even the oxide film in the underlying base is etched, adversely affecting the formation of a source-drain diffused layer.
On the other hand, the use of Cl.sub.2 gas makes it difficult to control the etching shape because of its small sidewall protective effect at the time of etching. Moreover, in order to carry out an anisotropic etching by the use of Cl.sub.2 gas it is necessary to perform the etching at low pressure and the selectivity has to be lowered. Furthermore, since the Cl.sub.2 gas even etches the photoresist film which is a mask, there is also a problem that a pattern formation with high accuracy cannot be obtained.
Finally, with the use of HBr gas, the selectivity with respect to the oxide film is a high value of over 100, and an etching with high anisotropy can be accomplished, but when the second polysilicon film is etched as in the above, a deposited film is formed on the side face parts of the wiring. Because of this, at the time of over etching filamentlike residuals are formed with this deposited film as a mask, becoming the cause of short-circuiting of the wiring or generation of dust and generating defects deteriorating the reliability of semiconductor device and the yield of manufacturing.